Field of the Invention
The present invention is directed in general to digital radio broadcast transmitters and receivers and methods for operating them. In one aspect, the present invention relates to methods and apparatus for blending digital and analog portions of an audio signal in a radio receiver.
Description of the Related Art
Digital radio broadcasting technology delivers digital audio and data services to mobile, portable, and fixed receivers using existing radio bands. One type of digital radio broadcasting, referred to as in-band on-channel (IBOC) digital radio broadcasting, transmits digital radio and analog radio broadcast signals simultaneously on the same frequency using digitally modulated subcarriers or sidebands to multiplex digital information on an AM or FM analog modulated carrier signal. HD Radio™ technology, developed by iBiquity Digital Corporation, is one example of an IBOC implementation for digital radio broadcasting and reception. With IBOC digital radio broadcasting, the audio signal can be redundantly transmitted on the analog modulated carrier and the digitally modulated subcarriers by transmitting the analog audio AM or FM backup audio signal (which is delayed by the diversity delay) so that the analog AM or FM backup audio signal can be fed to the audio output when the digital audio signal is absent, unavailable, or degraded. In these situations, the analog audio signal is gradually blended into the output audio signal by attenuating the digital signal such that the audio is fully blended to analog as the digital signal becomes unavailable. Similar blending of the digital signal into the output audio signal occurs as the digital signal becomes available by attenuating the analog signal such that the audio is fully blended to digital as the digital signal become available. However, due to limitations in the smoothness of the blending function, blend transitions between analog and digital signals can degrade the listening experience when the audio differences between the analog and digital signals are significant. For example, at the edge of a station coverage where the signal is changing around the minimum required level, squawk-like interference can occur when the signal is briefly under the required level, in which case the decoder fails to generate real audio, and instead sends out meaningless data. In most cases, this trash data tends to fluctuate to the maximum amplitude, thereby creating a sudden, short-period and uncomfortable squawk-like pop noise. Existing solutions to smooth the blending function require large audio packets buffers for storing decoded audio packets, adding cost and increased on-chip memory requirements for the receivers. Other solutions aim at reducing the frequency of blends by using computed estimated signal-to-noise values in the blending decision, but such estimated values have limited accuracy with certain conditions, such as with channels having interference or a moving automobile where there is selective fading channel experience in the mobile environment. Because of at least the demonstrated challenges for blending digital and analog signals at a radio receiver without noticeably impairing the listening experience, it would be desirable to have a more practical and cost effective solution for processing the digital audio.